Stimulation of the human granulocyte with soluble factors of phagocytosable particles initiates a marked increase of oxygen consumption with the concomitant production of oxygen metabolites: superoxide (O2-minus), hydroxyl radical (OH) and hydrogen peroxide (H2O2). We have partially isolated and characterized the enzyme responsible for this respiratory burst, an NADPH-dependent oxidase. The physicochemical properties of this oxidase have been further examined for its flavin constituent and possible linkage to a cytochrome b. The attempt to isolate the enzyme to homegeneity and make an antibody to the protein will enable studies directed at subcellular localization and the process of activation of the enzyme, as well as clinical and diagnostic studies of chronic granulomatous disease, a disorder which manifests no respiratory burst activity.